In recent years, technical progress in many industries including primarily the aircraft industry, created the need for the development of materials superior to conventional materials in physical and mechanical properties, such as, heat resistance at elevated temperatures.
It is generally known to improve, the physical properties of commentional material to reinforcing them with; carbon fibers; fibers of metals such as tungsten, molybdenum, and steel; composite fibers resulting from covering the surface of tungsten filaments with boron, silicon carbide, or the like; polycrystalline fibers of alumina, zirconia, and the like; and whiskers of silicon carbide and some other material.
Metal oxide fibers, one group of these reinforcements for composite materials, have characteristics such that they can be used in high temperature oxidative atmospheres wherein neither carbon fibers nor metallic fibers will be utilizable. Metal oxide fibers do not lose the superior mechanical property at high temperatures because of their generally high melting points. Accordingly, metal oxide fibers are expected to be used not only as compounding reinforcements but also over a wide range of applications in various industrial fields.
The present applicant has previously proposed processes for producing inorganic fibers, for example, alumina, alumina-silica, titania, and zirconia fibers, using polymetalloxane solutions of room temperature viscosities 1 to 5000 poises as starting materials (Japanese Patent Publication Nos. 12,736/76 and 13,768/76 and Japanese Patent Application Laid-Open Nos. 124,336/74, 136,424/75, and 18726/75).
The above processes have various advantages over other methods in that the metal oxide contents in precursor fibers are high and this results in dense fibers after baking, in that the fibers have high tenacity and high elasticity, and in that the spinning solutions exhibit good spinnabality because of their homogeneity.
However, the above processes have drawbacks in that fiber at least 2 cm in length cannot be attained when the polymetalloxane solutions are used as spinning solutions specially for producing short fibers of about 3 .mu.m in filament diameter. Also small filament diameters of 10 .mu.m and less are difficult to attain when producing continuous fibers. Therefore, it is desired to achieve better fiber stretchability.